Enkephalin is a major neurotransmitter expressed in regions throughout the body, and is the most prominent neurotransmitter mimicked by opiate drugs. The dopamine transporter is the site of action of cocaine reward/reinforcement in the brain. In efforts to understand the mechanisms involved in opiate and cocaine actions, tissue-specific expression and overexpression of the dopamine transporter and preproenkephalin were sought over this FY. In attempts to overexpress enkephalin, transgenic mice were created that contained the human enkephalin promoter region fused to a rat enkephalin cDNA. A parallel study utilized the identical promoter region in a construct designed to obtain overexpression of a rat preproenkephalin cDNA. Finally, mice with dopamine transporter expression driven by a tyrosine hydroxylase promoter were created. These mice were examined for detectable levels of overexpression, observed for behavioral differences from control mice with normal levels of expression, and studied to seek other signs of physiological disruption attributable to overexpression. Two to four fold overexpression above wild type levels was observed in the testes of enkephalin-transgenic animals. Modest mRNA expression was observed in several brain regions. Behaviorally, the male mice appeared to mate normally, but the fertility/ breeding success rate was reduced. Sperm motility was reduced for one male but appeared normal for others. One male had severely reduced testis size, and altered morphology. Studies of behaviors in the transgenic mice suggested that responses of enkephalin overexpressing animals to novel environments, opiate agonists and opiate antagonists were indistinguishable from nontransgenic controls. Similarly, no difference in locomotor behaviors or psychomotor stimulant responses was noted in animals expressing the dopamine transporter cDNA driven by the tyrosine hydroxylase promoter.